What is valve authority and why does it matter?
What Valve Authority Actually Means
Valve authority (N) is the single most important yet frequently overlooked concept in hydronic control system design. It is defined as:
**N = ΔP_valve / (ΔP_valve + ΔP_circuit)**
At design flow with the valve fully open, ΔP_valve is the pressure drop across the valve and ΔP_circuit is the sum of all other pressure drops in that branch — coil, piping, fittings, strainers, and balancing valves. When N = 0.50, the valve claims half the available branch pressure drop and the installed flow vs. stem-travel relationship closely mirrors the valve's inherent characteristic curve.
### Why Low Authority Destroys Control
When N falls below 0.25, the installed characteristic becomes severely distorted. An equal-percentage valve — designed to provide gentle, exponential flow increase with stem travel — begins to behave like a quick-opening valve, delivering most of its flow in the first 20% of stroke. Above 50% open, further movement produces almost no change in flow. The practical consequence: the BMS controller calls for a small cooling increase, the actuator opens 5%, and the coil suddenly floods with near-design flow, overshooting the temperature setpoint.
This distortion occurs because at low authority, the majority of the system pressure drop sits in the fixed circuit (coil, piping), not the valve. As the valve opens and its resistance drops, flow is limited by the fixed circuit, not the valve — the valve loses its ability to regulate.
### The 0.50 Target
ASHRAE Guideline 36 and the ASHRAE Handbook both recommend targeting N ≥ 0.50, with 0.25 as the absolute minimum. Achieving 0.50 requires the design engineer to allocate 50% of the branch ΔP to the control valve — which often means selecting a smaller valve (higher ΔP, lower Cv) than a line-size approach would suggest. In retrofit applications where N is already low, pressure-independent control valves (PICVs) from Belimo and Siemens provide an elegant remedy by decoupling valve authority from the system hydraulics entirely — the PICV's internal pressure regulator maintains constant flow for any stem position regardless of upstream pressure fluctuations, effectively delivering N = 1.0.
Valve Authority Impact on Controllability
How valve authority (N) affects installed flow characteristic and practical controllability in hydronic HVAC systems.
| Authority (N) | Installed Characteristic | Stem Travel for 90% Flow | Controllability | Recommendation |
|---|---|---|---|---|
| N ≥ 0.50 | Matches inherent curve closely | ~65–75% stem | Excellent — predictable and stable | Target for all modulating valves |
| 0.25 ≤ N < 0.50 | Moderate distortion | ~45–60% stem | Acceptable — PID tuning can compensate | Minimum acceptable range |
| 0.10 ≤ N < 0.25 | Severe distortion | ~25–40% stem | Poor — valve acts near quick-opening | Avoid; use PICV or smaller valve |
| N < 0.10 | No effective control | <20% stem | Unusable — valve is essentially on/off | Must redesign or retrofit with PICV |
| N = 1.0 (PICV) | True inherent characteristic | Per manufacturer curve | Ideal — independent of system ΔP | Best-in-class, higher first cost |
🔑 Key Takeaways
- ✓Valve authority N = ΔP_valve / (ΔP_valve + ΔP_circuit); target N ≥ 0.50 for excellent control
- ✓Below N = 0.25, equal-percentage valves distort into quick-opening behaviour — avoid this range
- ✓Low authority causes the valve to deliver most flow in the first 20% of stroke, making precise control impossible
- ✓Pressure-independent control valves (PICVs) eliminate authority concerns by regulating flow at any stem position
- ✓If a valve never exceeds 50% open at design conditions, it is almost certainly oversized with poor authority